Major refactor to push generic methods to other packages

[kescobo]

Removing DistanceMatrix and PCoA types and Methods in favor of Distances.jl and MultivariateStats.jl

Types of changes

This PR implements the following changes:
(Please tick any or all of the following that are applicable)

• 🐛 Bug fix (A non-breaking change, which fixes an issue).
• 💥 Breaking change (fix or feature that would cause existing functionality to change).

📋 Additional detail

• No longer create DistanceMatrix type
  • Instead of getdm(::AbundanceTable, BrayCurtis()), will use pairwise(BrayCurtis(), ::AbundanceTable). Depends on Add pairwise() method from Distances.jl SpatialEcology.jl#36
• No longer create PCoA type
  • Instead, use MDS from MultivariateStats.jl. Depends on New MDS API JuliaStats/MultivariateStats.jl#85
  • eg instead of pcoa(::DistanceMatrix), use fit(::MDS, ::Array{Number, 2})
  • Plot recipes available from Add ordinations (MDS and PCA) JuliaPlots/StatsPlots.jl#230
• No longer provide optimal leaf ordering methods for HClust - moved to Clustering.jl (see here)
• Update documentation
  • include more examples
  • add more docstrings

☑️ Checklist

• 🎨 The changes implemented is consistent with the julia style guide.
• 📘 I have updated and added relevant docstrings, in a manner consistent with the documentation styleguide.
• 📘 I have added or updated relevant user and developer manuals/documentation in docs/src/.
• 🆗 There are unit tests that cover the code changes I have made.
• 🆗 The unit tests cover my code changes AND they pass.
• 📝 I have added an entry to the [UNRELEASED] section of the manually curated CHANGELOG.md file for this repository.
• 🆗 All changes should be compatible with the latest stable version of Julia.
• 💭 I have commented liberally for any complex pieces of internal code.

[kescobo]

Merging, though can't be released while waiting on several other packages to make releases.